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1.
M.H. Moore  R.L. Hudson 《Icarus》2003,161(2):486-500
Infrared spectra and radiation chemical behavior of N2-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N2-rich ices containing CH4 and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH2N2) form in the solid phase, along with several radicals. NH3 is also identified in irradiated N2 + CH4 and N2 + CH4 + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N2/CH4 ratios from 100 to 4, and in three-component mixtures have an initial N2/(CH4 + CO) ratio of 50. HCN and HNC are not detected in N2-dominated ices when CH4 is replaced with C2H6, C2H2, or CH3OH.The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N2 + CH4 and N2 + CH4 + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N2-rich ices containing CH4.We examine the evolution of spectral features due to acid-base reactions (acids such as HCN, HNC, and HNCO and a base, NH3) triggered by warming irradiated ices from 12 K to 30-35 K. We identify anions (OCN−, CN−, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH3, NH4OCN, and NH4CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides.  相似文献   

2.
We calculate the amount of methane that may form via reactions catalyzed by metal-rich dust that condenses in the wake of large cometary impacts. Previous models of the gas-phase chemistry of impacts predicted that the terrestrial planets' atmospheres should be initially dominated by CO/CO2, N2, and H2O. CH4 was not predicted to form in impacts because gas-phase reactions in the explosion quench at temperatures ∼2000 K, at which point all of the carbon is locked in CO. We argue that the dust that condenses out in the wake of a large comet impact is likely to have very effective catalytic properties, opening up reaction pathways to convert CO and H2 to CH4 and CO2, at temperatures of a few hundred K. Together with CO2, CH4 is an important greenhouse gas that has been invoked to compensate for the lower luminosity of the Sun ∼4 Gyr ago. Here, we show that heterogeneous (gas-solid) reactions on freshly-recondensed dust in the impact cloud may provide a plausible nonbiological mechanism for reducing CO to CH4 before and during the emergence of life on Earth, and perhaps Mars as well. These encouraging results emphasize the importance of future research into the kinetics and catalytic properties of astrophysical condensates or “smokes” and also more detailed models to determine the conditions in impact-generated dust clouds.  相似文献   

3.
Two different simulation experiments of prebiotic synthesis were carried out in a CH4/N2/H2 atmosphere with spark discharge activation of aqueous aerosols and liquid water. In both cases, a hydrophilic tholin and a hydrophobic tholin were obtained. The methodology developed by our group for the characterisation of hydrophilic tholins [Ruiz-Bermejo, M., Menor-Salván, C., Mateo-Martí, E., Osuna-Esteban, S., Martín-Gago, J.A., Veintemillas-Verdaguer, S., 2008. Icarus 198, 232-241] was used in order to study the hydrophobic tholins. The gas precursors of the tholins from mixtures containing CH4, with and without H2, were studied. We propose that the formation of the hydrophobic tholins involves the formation of unsaturated oligomeric hydrocarbon chains from vinyl and acetylene monomers, as well as allene derivatives formed in the gas phase after the incorporation of polar groups into these hydrocarbon chains. Finally, we compare our results concerning hydrophobic tholins with HCN polymers, since it is generally suggested that the polymeric material formed in spark experiments are possible oligomers of HCN, and that Titan’s tholins could be poly-HCN.  相似文献   

4.
The reactivity of C2(X1Σ+g) with simple saturated (CH4, C2H6 and C3H8) and unsaturated (C2H2 and C2H4) hydrocarbons has been studied in the gas phase over the temperature range 24-300 K using the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique. All reactions have been found to be very rapid in this temperature range and the rate coefficients are typically ?10−10 cm3 molecule−1 s−1 with the exception of methane for which the rate coefficient is one order of magnitude lower: ∼10−11 cm3 molecule−1 s−1. These results have been analyzed in terms of potential destruction sources of C2(X1Σ+g) in the atmospheres of Titan and the Giant Planets. It appears that the rate coefficient of the reaction 1C2 + CH4 should be updated with our new data and that reactions with C2H2, C2H4 and C2H6 should also be included in the existing photochemical models.  相似文献   

5.
Sang J. Kim  T.R. Geballe  J.H. Kim 《Icarus》2009,202(1):354-357
Jupiter exhibits bright H+3 auroral arcs at 3-4 microns that cool the hot (>1000 K) ionosphere above the ∼10−7 bar level through the infrared bands of this trace constituent. Below the 10−7 bar level significant cooling proceeds through infrared active bands of CH4, C2H2, and C2H6. We report the discovery of 3-micron line emission from these hydrocarbon species in spectra of the jovian south polar region obtained on April 18 and 20, 2006 (UT) with CGS4 on the United Kingdom Infrared Telescope. Estimated cooling rates through these molecules are 7.5×10−3, 1.4×10−3, and , respectively, for a total nearly half that of H+3. We derive a temperature of 450 ± 50 K in the 10−7-10−5 bar region from the C2H2 lines.  相似文献   

6.
The Electron Spectrometer (ELS), one of the sensors making up the Cassini Plasma Spectrometer (CAPS) revealed the existence of numerous negative ions in Titan's upper atmosphere. The observations at closest approach (∼1000 km) show evidence for negatively charged ions up to ∼10,000 amu/q, as well as two distinct peaks at 22±4 and 44±8 amu/q, and maybe a third one at 82±14 amu/q. We present the first ionospheric model of Titan including negative ion chemistry. We find that dissociative electron attachment to neutral molecules (mostly HCN) initiates the formation of negative ions. The negative charge is then transferred to more acidic molecules such as HC3N, HC5N or C4H2. Loss occurs through associative detachment with radicals (H and CH3). We attribute the three low mass peaks observed by ELS to CN−, C3N−/C4H− and C5N−. These species are the first intermediates in the formation of the even larger negative ions observed by ELS, which are most likely the precursors to the aerosols observed at lower altitudes.  相似文献   

7.
Tholins are complex organic materials produced by irradiation of several carbon and nitrogen rich atmospheres. They could have played a key role in the origin of life on Earth because their ability to release important bioorganics, which are nowadays present in proteins, nucleic-acid bases and other important biostructures. Usually, the yield of these compounds is higher after acid hydrolysis, however little is known about the structure and chemical composition of the tholins. In this work, we propose the use of different spectroscopic and separation techniques, which are not usually applied in this field, in order to obtain complete information about the tholin structure and behavior. Two different simulation experiments of prebiotic synthesis were carried out in CH4/N2/H2 atmosphere out from spark discharge activation of aqueous aerosols and liquid water, respectively. In both cases, a hydrophilic tholin and a hydrophobic tholin were obtained. Herein, we report the application of this methodology to our hydrophilic tholins and we review, briefly, some astrobiological aspects related to these complex substances.  相似文献   

8.
The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N2, CH4, H, H2, 3CH2, CH3, C2H4, C2H5, C2H6, N(4S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N2, CH4, and H2. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N2 and CH4 INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be and .  相似文献   

9.
Galina M. Chaban 《Icarus》2007,187(2):592-599
An absorption band at ∼4.26 μm wavelength attributed to the asymmetric stretching mode of CO in CO2 has been found on two satellites of Jupiter and several satellites of Saturn. The wavelength of pure CO2 ice determined in the laboratory is 4.2675 μm, indicating that the CO2 on the satellites occurs either trapped in a host material, or in a chemical or physical complex with other materials, resulting in a blue shift of the wavelength of the band. In frequency units, the shifts in the satellite spectra range from 3.7 to 11.3 cm−1. We have performed ab initio quantum chemical calculations of CO2 molecules chemically complexed with one, two, and more H2O molecules and molecules of CH3OH to explore the possibility that the blue shift of the band is caused by chemical complexing of CO2 with other volatile materials. Our computations of the harmonic and anharmonic vibrational frequencies using high levels of theory show a frequency shift to the blue by 5 cm−1 from pure CO2 to CO-H2O, and an additional 5 cm−1 from CO2-H2O to CO2-2H2O. Complexing with more than two H2O molecules does not increase the blue shift. Complexes of CO2 with one molecule of CH3OH and with one CH3OH plus one H2O molecule produce smaller shifts than the CO2-2H2O complex. Laboratory studies of CO2:H2O in a solid N2 matrix also show a blue shift of the asymmetric stretching mode.  相似文献   

10.
The combined observations of Saturn's moon Enceladus by the Cassini CAPS, INMS and UVIS instruments detected water vapor geysers in which were present molecular nitrogen (N2), carbon dioxide (CO2), methane (CH4), propane (C3H8), acetylene (C2H2), and several other species, together with all of the decomposition products of water. We propose that the presence of N2 in the plume indicates thermal decomposition of ammonia, and hence high temperatures in the interior of the moon (e.g., 500 to 800 K). Such an environment also appears to be suitable for the production of methane (CH4) from carbon monoxide (CO), or carbon dioxide (CO2). The presence of C2H2 and C3H8 strongly suggest that catalytic reactions took place within a very hot environment. The internal environment of Enceladus is inferred to be or have been favorable for aqueous, catalytic chemistry. This permits the synthesis of many complex organic compounds that could be detected in future Cassini observations.  相似文献   

11.
Ethane (C2H6), methylacetylene (CH3C2H or C3H4) and diacetylene (C4H2) have been discovered in Spitzer 10-20 μm spectra of Uranus, with 0.1-mbar volume mixing ratios of (1.0±0.1)×10−8, (2.5±0.3)×10−10, and (1.6±0.2)×10−10, respectively. These hydrocarbons complement previously detected methane (CH4) and acetylene (C2H2). Carbon dioxide (CO2) was also detected at the 7-σ level with a 0.1-mbar volume mixing ratio of (4±0.5)×10−11. Although the reactions producing hydrocarbons in the atmospheres of giant planets start from radicals, the methyl radical (CH3) was not found in the spectra, implying much lower abundances than in the atmospheres of Saturn or Neptune where it has been detected. This finding underlines the fact that Uranus' atmosphere occupies a special position among the giant planets, and our results shed light on the chemical reactions happening in the absence of a substantial internal energy source.  相似文献   

12.
Ronen Jacovi 《Icarus》2008,196(1):302-304
Titan's haze, formed by photolysis of C2H2, C2H4 and HCN, was found experimentally to trap Ar, Kr and Xe with efficiencies of 3.5 × 10−4, 1.9 × 10−3 and 6.5 × 10−2 [noble gas atom]/[carbon atom] in the polymer, respectively. The rate of aerosol formation and settling down of 3 × 10−13 kg m−2 s−1, as inferred from our experiments on CH4 photolysis in the far UV [Podolak, M., Bar-Nun, A., 1979. Icarus 39, 272-276], is sufficient to reduce the mixing ratios of 36Ar and 40Ar to their low values of (2.8 ± 0.3) × 10−7 and (4.3 ± 0.1) × 10−3, respectively, and those of Kr and Xe to below the detection limit of 10−8.  相似文献   

13.
A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH43.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH47.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4molecule. Partial thermalization of absorbed solar radiation in the CH43.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km−1forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2individually as the major constituent. If a steep temperature gradient ∼20 K km−1is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4mixing ratio to attain a calculated ratio ofT/(= 3.5 K amu−1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2as the dominant constituent. One possibility is a 3 μbar N2atmosphere with 0.3% CH4that has 106 K isothermal region (T/= 3.8 K amu−1) and ∼8 K km−1surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH4density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.).  相似文献   

14.
Simon Petrie 《Icarus》2004,171(1):199-209
We report results of quantum chemical calculations of Mg+/ligand bond dissociation energies involving ligands identified as major constituents of Titan's upper atmosphere. Trends identified in these results allow elucidation of the important bimolecular and termolecular reactions of Mg+, and of simple molecular ions containing Mg+, arising from meteoric infall into Titan's atmosphere. Our study highlights, and includes calculated rate coefficients for, crucial ligand-switching and ligand-stripping reactions which ensure that a dynamic equilibrium exists between atomic and molecular ions of Mg+. Neutralization of ionized meteoric Mg is expected to produce the radical MgNC in high yield. The highly polar MgNC radical should provide an excellent nucleation site for condensation of polar (e.g., HCN, CH3CN, and HC3N) and highly unsaturated (e.g., C2H2, C4H2, and C2N2) neutrals at comparatively high altitude, leading to precipitation of Mg-doped tholin-like material. The implications for Titan's prebiotic chemical evolution, of the surface deposition of such material (which may feasibly contain magnesium porphyrins, or other bioactive Mg-containing complexes) remain to be assessed.  相似文献   

15.
We present new experimental results on impact shock chemistry into icy satellites of the outer planets. Icy mixtures of pure water ice with CO2, Na2CO3, CH3OH, and CH3OH/(NH4)2SO4 at 77 K were ablated with a powerful pulsed laser—a new technique used to simulate shock processes which can occur during impacts. New products were identified by GC-MS and FTIR analyses after laser ablation. Our results show that hydrogen peroxide is formed in irradiated H2O/CO2 ices with a final concentration of 0.23%. CO and CH3OH were also detected as main products. The laser ablation of frozen H2O/Na2CO3 generates only CO and CO2 as destruction products from the salt. Pulsed irradiation of water ice containing methanol leads also to the formation of CO and CO2, generates methane and more complex molecules containing carbonyl groups like acetaldehyde, acetone, methyl formate, and a diether, dimethyl formal. The last three compounds are also produced when adding ammonium sulfate to H2O/CH3OH ice, but acetone is more abundant. The formation of two hydrocarbons, CH4 and C2H6 is observed as well as the production of three nitrogen compounds, nitrous oxide, hydrogen cyanide, and acetonitrile.  相似文献   

16.
Sang J. Kim  T.R. Geballe  A. Jung  Y.C. Minh 《Icarus》2010,208(2):837-849
We present latitudinally-resolved high-resolution (R = 37,000) pole-to-pole spectra of Jupiter in various narrow longitudinal ranges, in spectral intervals covering roughly half of the spectral range 2.86-3.53 μm. We have analyzed the data with the aid of synthetic spectra generated from a model jovian atmosphere that included lines of CH4, CH3D, NH3, C2H2, C2H6, PH3, and HCN, as well as clouds and haze. Numerous spectral features of many of these molecular species are present and are individually identified for the first time, as are many lines of and a few unidentified spectral features. In both polar regions the 2.86-3.10-μm continuum is more than 10 times weaker than in spectra at lower latitudes, implying that in this wavelength range the single-scattering albedos of polar haze particles are very low. In contrast, the 3.24-3.53 μm the weak polar and equatorial continua are of comparable intensity. We derive vertical distributions of NH3, C2H2 and C2H6, and find that the mixing ratios of NH3 and C2H6 show little variation between equatorial and polar regions. However, the mixing ratios of C2H2 in the northern and southern polar regions are ∼6 and ∼3 times, respectively, less than those in the equatorial regions. The derived mixing ratio curves of C2H2 and C2H6 extend up to the 10−6 bar level, a significantly higher altitude than most previous results in the literature. Further ground-based observations covering other longitudes are needed to test if these mixing ratios are representative values for the equatorial and polar regions.  相似文献   

17.
Powdered samples of carbon-nitrogen-hydrogen “tholins” that mimic Titan's atmosphere aerosols were produced under levitation conditions in the laboratory with a dusty plasma (PAMPRE experiment) using different initial N2:CH4 gas mixtures and studied using UV Raman and infrared spectroscopy, X-ray diffraction and high resolution transmission electron microscopy (HRTEM). Comparison between the tholins produced in the PAMPRE experiments and samples prepared by other techniques reveal that they form a fairly homogeneous family of hydrogenated carbon nitride materials. Wall effects during the PAMPRE deposition experiments and other were found to have little effect on the chemical structure of tholins. The first-order UV Raman bands of the disordered carbonaceous materials point to a large contribution of sp2 clusters present compared with olefinic CN or CC groupings, whereas features at 690 and 980 cm−1 suggest C3N3 rings are present as a species inserted in the macromolecular network. Diffraction techniques do not indicate the presence of large polyaromatic species in any of the tholins studied, whatever their nitrogen concentration, in disagreement with certain previous observations. This precludes the idea that the nature and degree of absorption in the visible range is controlled by the size of polyaromatic species, as has been observed in series of carbon-based materials obtained via thermal processing. Infrared spectroscopy analysis of the tholins has confirmed earlier identifications of chemical groups present including primary amines, nitriles, and alkyl moieties such as CH2/CH3, but has ruled out CH2/CH3 branches appearing on secondary or tertiary amines. Similar analyses were also performed on a polymeric (HCN)x material, which was found to present several similarities with the tholins, hence suggesting similar polymerization processes.  相似文献   

18.
Observations of Jupiter by Cassini/CIRS, acquired during the December 2000 flyby, provide the latitudinal distribution of HCN and CO2 in Jupiter's stratosphere with unprecedented spatial resolution and coverage. Following up on a preliminary study by Kunde et al. [Kunde, V.G., and 41 colleagues, 2004. Science 305, 1582-1587], the analysis of these observations leads to two unexpected results (i) the total HCN mass in Jupiter's stratosphere in 2000 was (6.0±1.5)×1013 g, i.e., at least three times larger than measured immediately after the Shoemaker-Levy 9 (SL9) impacts in July 1994 and (ii) the latitudinal distributions of HCN and CO2 are strikingly different: while HCN exhibits a maximum at 45° S and a sharp decrease towards high Southern latitudes, the CO2 column densities peak over the South Pole. The total CO2 mass is (2.9±1.2)×1013 g. A possible cause for the HCN mass increase is its production from the photolysis of NH3, although a problem remains because, while millimeter-wave observations clearly indicate that HCN is currently restricted to submillibar (∼0.3 mbar) levels, immediate post-impact infrared observations have suggested that most of the ammonia was present in the lower stratosphere near 20 mbar. HCN appears to be a good atmospheric tracer, with negligible chemical losses. Based on 1-dimensional (latitude) transport models, the HCN distribution is best interpreted as resulting from the combination of a sharp decrease (over an order of magnitude in Kyy) of wave-induced eddy mixing poleward of 40° and an equatorward transport with velocity. The CO2 distribution was investigated by coupling the transport model with an elementary chemical model, in which CO2 is produced from the conversion of water originating either from SL9 or from auroral input. The auroral source does not appear adequate to reproduce the CO2 peak over the South Pole, as required fluxes are unrealistically high and the shape of the CO2 bulge is not properly matched. In contrast, the CO2 distribution can be fit by invoking poleward transport with a velocity and vigorous eddy mixing (). While the vertical distribution of CO2 is not measured, the combined HCN and CO2 results imply that the two species reside at different stratospheric levels. Comparing with the circulation regimes predicted by earlier radiative-dynamical models of Jupiter's stratosphere, and with inferences from the ethane and acetylene stratospheric latitudinal distribution, we suggest that CO2 lies in the middle stratosphere near or below the 5-mbar level.  相似文献   

19.
We have analyzed Titan observations performed by the Infrared Space Observatory (ISO) in the range 7-30 μm. The spectra obtained by three of the instruments on board the mission (the short wavelength spectrometer, the photometer, and the camera) were combined to provide new and more precise thermal and compositional knowledge of Titan’s stratosphere. With the high spectral resolution achieved by the SWS (much higher than that of the Voyager 1 IRIS spectrometer), we were able to detect and separate the contributions of most of the atmospheric gases present on Titan and to determine disk-averaged mole fractions. We have also tested existing vertical distributions for C2H2, HCN, C2H6, and CO2 and inferred some information on the abundance of the first species as a function of altitude. From the CH3D band at 1161 cm−1 and for a CH4 mole fraction assumed to be 1.9% in Titan’s stratosphere, we have obtained the monodeuterated methane-averaged abundance and retrieved a D/H isotopic ratio of 8.7−1.9+3.2 × 10−5. We discuss the implications of this value with respect to current evolutionary scenarios for Titan. The ν5 band of HC3N at 663 cm−1 was observed for the first time in a disk-averaged spectrum. We have also obtained a first tentative detection of benzene at 674 cm−1, where the fit of the ISO/SWS spectrum at R = 1980 is significantly improved when a constant mean mole fraction of 4 × 10−10 of C6H6 is incorporated into the atmospheric model. This corresponds to a column density of ∼ 2 × 1015 molecules cm−2 above the 30-mbar level. We have also tested available vertical profiles for HC3N and C6H6 and adjusted them to fit the data. Finally, we have inferred upper limits of a few 10−10 for a number of molecules proposed as likely candidates on Titan (such as allene, acetonitrile, propionitrile, and other more complex gases).  相似文献   

20.
The 14 May 1971 occultation β Scorpi C by Io was successfully observed in ultraviolet light near Kingston, Jamaica. Within the limits established by time resolution and the signal-to-noise ratio, both the disappearance and reappearance were found to be instantaneous. Upper limits for the surface pressure for N2, CH4, and H2 atmospheres are 0.09, 0.13, and 44.ubar, respectively. The corresponding number densities are 6.2 × 1012, 9.4 × 1012, and 3.2 × 1015 cm−3. An isothermal atmosphere at a temperature of 100°K was assumed.  相似文献   

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